Apparatus for etching or depositing a desired profile onto a surface

ABSTRACT

An apparatus and method for modifying the surface of an object by contacting said surface with a liquid processing solution using the liquid applicator geometry and Marangoni effect (surface tension gradient-driven flow) to define and confine the dimensions of the wetted zone on said object surface. In particular, the method and apparatus involve contouring or figuring the surface of an object using an etchant solution as the wetting fluid and using real-time metrology (e.g. interferometry) to control the placement and dwell time of this wetted zone locally on the surface of said object, thereby removing material from the surface of the object in a controlled manner. One demonstrated manifestation is in the deterministic optical figuring of thin glasses by wet chemical etching using a buffered hydrofluoric acid solution and Marangoni effect.

RELATED APPLICATION

This application is a division of U.S. application Ser. No. 09/687,775,filed Oct. 13, 2000 now U.S. Pat. No. 6,555,017, entitled “SurfaceContouring By Controlled Application Of Processing Fluid Using MarangoniEffect.”

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to modifying the surface of an object bycontacting said surface with a liquid processing solution using theliquid applicator geometry and the Marangoni effect (surface tensiongradients) to define and confine the dimensions of the wetted zone onsaid object surface. In particular, the invention relates to contouringor figuring the surface of an object using an etchant solution as thewetting fluid and using real-time metrology (e.g. interferometry) tocontrol the placement and dwell time of this wetted zone locally on thesurface of said object, thereby removing material from the surface ofthe object in a controlled manner. One demonstrated manifestation ofthis invention is in the deterministic optical figuring of thin glassesby wet chemical etching using a buffered hydrofluoric acid solution andthe Marangoni effect.

2. Description of Related Art

Small-tool finishing or figuring of optical surfaces basically involvesmoving a small polishing tool in a controlled manner to shape thesurface of an optic. It is a critical technology for producing opticsfor applications ranging from camera lenses for the consumer market, tolarge-aperture optics for inertial confinement fusion and spacetelescope systems. Examples of optics figured by these techniquesinclude complex-figured aspheric lenses, continuous contour phaseplates, Alvarez lenses, and optics requiring local figure correctionafter processing via traditional lap polishing. Small-tool finishing issimultaneously a mature technology and one undergoing continuousdevelopment driven by the high cost and reproducibility problems ofprocesses when applied to ever-tightening figure tolerances.

Traditional techniques employ rotary polishing pads. Recent developments(e.g. U.S. Pat. No. 5,591,098 and references therein) utilize directedflow fields to impinge fine abrasive slurries onto the optical surface.Magnetorheological finishing (e.g. U.S. Pat. Nos. 5,499,313, 5,795,212,5,839,944, 5,951,369) extend this concept by controlling the viscosityof specially formulated abrasive slurry by application of magneticfields. Ion-beam milling techniques (e.g. U.S. Pat. Nos. 3,988,564,5,969,368) are alternative, fundamentally different methods forhigh-accuracy optical figuring. All of the above-mentioned techniquessuffer fundamental limitations. They rely on process knowledge ofremoval rates, and are therefore iterative processes: the workpiece mustbe dismounted from the machine and measured, reworked and remeasured,until specifications are met. Ion beam milling techniques require large,expensive vacuum processing chambers and are not applicable to allmaterials. Abrasive small-tool polishing techniques cannot be used tofigure very thin optics since the local mechanical stresses involvedcause workpiece deformations that impact removal control and can evencause breakage.

Material removal on optical surfaces can be accomplished by etching ordissolution methods (e.g. silicate glasses are soluble in hydrofluoricacid solutions), but until now, wet etching has not been employed tofigure optics. The problem has been largely how to confine the wettedzone of etchant solution to a specific, stable geometry. A surface beingetched is hydrophilic to the etching solution. If a bolus of etchantsolution is moved along the surface of a workpiece, a thin liquid filmwill be left behind that will continue to etch the surface. Recently(U.S. Pat. No. 5,660,642), surface-tension gradient driven flow (theMarangoni effect) has been shown to be effective in causing this thinentrained film to flow off the surface of a workpiece back into the bulkliquid if said liquid is applied in the appropriate manner. The presentinvention utilizes the Marangoni effect to confine a wetted zone to astable, defined geometry on the surface of a workpiece. The wetted zoneremains of constant size and shape as it is moved around on theworkpiece surface. This allows the fluid to act on the surface of theworkpiece only in this wetted zone with no mechanical contact or inducedmechanical stresses applied to the workpiece. This in turn allows forreal-time local metrology of the processing, and allows for theprocessing of very thin 1 mm-thickness) plates that cannot be processedby other means.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for modifying the surface of an object by contacting thesurface of said object with a zone of liquid processing fluid such as anetching or deposition solution, and confining this zone by means ofsurface tension gradients (Marangoni effect). The size and shape of thiswetted zone is defined by the mechanical design of the liquid applicatorand the Marangoni effect. Additionally, this confined wetted zone canbe, if desired, moved relative to the object surface in a controlledfashion so as to generate a pattern on the surface.

It is a further object of this invention to provide a method andapparatus for using real-time metrology and control algorithms to movethis wetted zone in a deterministic fashion on the surface of the objectfor control of the figuring process.

It is a further object of this invention to produce large (>5 cm majoraxis) thin (<1 mm thickness) glass sheets figured flat to opticaltolerances by use of the above methods and apparatus.

A key feature of this invention is the establishment of a meniscus of anaqueous processing liquid on the surface of the object to be processed,simultaneous with a means to establish a surface tension gradient in themeniscus region, such that the surface tension of said liquid is weakestwhere the liquid attaches to the solid. The means to achieve this hasbeen reduced to practice by issuing the liquid upward and out of avertical tube placed in proximity to a downward facing surface, suchthat a circular liquid wetted zone is established on said surface andthe liquid flows down the outside surface of the tube. The liquid in themeniscus near its zone of attachment to the solid surface is relativelyquiescent, while the liquid in the meniscus region where it transitionsto the free surface falling film flow is continuously refreshed withliquid being pumped out of the supply orifice (tube). A surface tensiongradient can be established in this meniscus by introducing a volatileorganic compound (voc) into the vicinity of this meniscus, if such vochas the property that it is at least slightly water-soluble and producesa large reduction in the surface tension of water when dissolved inminute concentrations. Ethanol and isopropanol are examples of suchvoc's that are of relatively minor hazard.

Higher concentrations of this voc are absorbed into the meniscus zoneadjacent to the solid surface than are absorbed in the vicinity of thecontinuously refreshed zone near the free surface flow. Thus, thesurface tension of the liquid meniscus is lower near the solid surfacethan near the flowing free surface. This surface tension gradient issufficiently strong to cause a flow in the meniscus region away from thesolid surface toward the falling film. This flow prevents the wicking ofthe aqueous processing fluid onto a hydrophilic surface, and prevents athin film from being deposited as the wetted zone is moved slowlyrelatively to the workpiece surface while it is attached to saidsurface.

The voc can be introduced into the vicinity of the meniscus byevaporation of a pool of the liquid phase or from a porous mediumsaturated with the liquid phase of the voc. Alternatively, it can beintroduced by forced convection of a carrier gas, containing a smallamount of the voc, into this vicinity. The voc absorbed into the aqueousprocessing fluid can be removed by absorption on an activated carbonfilter medium, for example, or can be allowed to build up withoutdecreasing effectiveness for long periods when a sufficiently largeprocessing liquid reservoir is used.

Surface tension gradients can also be established by thermal gradients.Heating the workpiece, for example, can lower the surface tension of theliquid at the attached meniscus in the vicinity of the workpiece surfacecompared with the surface tension of the cooler fluid flowing at thefree surface flow. The resultant surface tension gradient is equivalentto that chemically produced according to the above description, and willresult in an equivalent flow.

Contact of the processing fluid (such as an etchant solution) in themanner described above does not require mechanical contact with theworkpiece and does not generate any mechanical stresses in theworkpiece. When applied to etching of polished glass or crystal surfacesfor optical application, a backpressure need not be applied to thebackside of the workpiece, and it can be left open. This allows forback-side monitoring of the workpiece by such means as interferometry,whereby the local workpiece thickness can be measured and the resultantdata used for real-time control of the motion of the wetted zonerelative to the workpiece to achieve a desired figure. Also, the lack ofmechanical stress generated by this process enables the figuring tooptical tolerances of very thin extruded glass sheets that are verydifficult or impossible to figure by traditional techniques.

The object of this invention is not limited to figuring of optics byetching, but can be applied to processes in which deposition occurs ontoa surface from an aqueous solution, such as epitaxial layer growth fromsolution onto substrates. It can also be used for local treatment ofsurfaces with aqueous solutions while keeping adjacent areas on thesurface dry. One skilled in the art will find other uses for the generalconcept.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a detailed view of the meniscus region of flow geometryutilized in one embodiment of the invention.

FIG. 2 shows a circular small-tool processing geometry wherein theprocessing fluid issues from the inside of a tube and flows down theoutside of said tube.

FIG. 3 shows a cross-section of a linear small-tool processing geometryof FIG. 1 wherein the processing fluid issues from a slot and flows downboth sides of a die forming the slot.

FIG. 4 shows an optical setup using a laser and associated optics togenerate an interference pattern between the two surfaces of a workpiecetransparent to the laser wavelength. This interference pattern is usedto calculate the thickness of the workpiece in the region illuminated bythe laser.

FIG. 5 shows an apparatus for etching a glass plate or similar material,consisting of a processing fluid applicator, processing fluidrecirculation system, voc delivery system, x-y translation stage,substrate holder, interferometer, and computer control system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves modifying the surface of an object bycontacting the surface with a liquid processing solution using theliquid applicator geometry and surface tension gradients (the Marangonieffect) to define and confine the dimensions of the wetted zone on theobject surface. More specifically, the invention is directed to a methodand apparatus for contouring or figuring the surface of an object usingan etchant solution as the wetting fluid and using real-time metrology(e.g. interferometry) to control the placement and dwell time of thiswetted zone, thereby removing material from the surface in a controlledmanner. The wetted zone remains a constant size and shape as it is movedaround the workpiece surface, whereby the etchant solution acts on thesurface only in this wetted zone. This allows for the processing of verythin (<1) mm thickness) plates, as well as the production of large(<5-cm major axis) plates, that cannot be processed by other apparatusor methods. This is accomplished by the establishment of a meniscus ofan aqueous liquid processing fluid (etchant solution) on the surface ofthe object being processed, such that one edge of the meniscus is theline attachment of the liquid to the surface, and the other edge becomesthe free surface of a falling film of the fluid. A surface tensiongradient (the Marangoni effect) is established in the meniscus byintroducing a volatile organic compound (voc) into the vicinity of themeniscus, if such voc has the property that it is at least slightlywater soluble and produces a large reduction in the surface tension ofwater when dissolved in minute concentrations. The voc may, for examplebe, ethanol or isopropanol. The surface tension gradients can also beestablished by thermal gradients, such as heating the workpiece. Thewetted zone may be established by pumping the processing fluid through aslit or annular orifice, wherein wetted zone takes the shape of a thinrectangular zone or of a circular spot.

Referring now to the drawings, FIG. 1 illustrates an applicator assemblygenerally indicated at 10 which includes an inlet fluid reservoir 11 anda slot die or orifice 12. Processing liquid 13 flows from reservoir 11through orifice 12 and down the outer side of the die 14 forming theorifice, and collects in an outer reservoir 15. A pump 16 pumpsprocessing fluid from reservoir 15 through line 17 back into reservoir11 through line 18 to continuously recirculate it. An inline filter 19can be installed in line 18. The orifice 12 may be circular, slotted orof a variety of configurations. The processing fluid 13 issuing out ofthe orifice at 20 is attached by capillary forces to the lower surface21 of a workpiece or member 22, creating a wetted zone 23 on saidworkpiece, bordered on all sides by a meniscus 24. The liquid 13 beingpumped through orifice 12 flows down the outside of the die 14 in afalling film flow. The member 25 surrounding the outer reservoir 15contains an another channel or reservoir 26 surrounding the outersurface of the die 14. This reservoir contains a suitable voc material27, such as a few milliliters of isopropanol. This voc 27 evaporatesinto the air space indicated at 28, surrounding the meniscus 24 of thewetted zone 23. The voc 27 absorbs in the liquid film, lowering itssurface tension at point 29 relative to point 30. The resultant surfacetension gradient pulls liquid away from point 29 and prevents the spreador entrainment of a thin liquid film on the workpiece surface 21. Thewetted zone 23 is thusly stably confined and can be translated relativeto the lower surface 21 of the workpiece 22.

The processing liquid or etchant solution 13 very slowly accumulatesdissolved solvent, but can remain viable for very many processing stepssince the concentration of solvent in the bulk liquid builds veryslowly, while the concentration in the very thin water film directlyabove the solvent vapor source is always relatively high. This solventcan alternatively be removed from the stream by absorption on, forexample, activated carbon filter media.

The surface tension gradient can also be realized by convection ofvoc-laden carrier gas into the meniscus region, or by convection ofheated gas in this vicinity, or by heating the backside of theworkpiece.

FIG. 2 illustrates an embodiment of the invention utilizing concentrictubes, wherein the wetted zone forms a circular region 31 defined byissuance of the processing liquid 13 from the inside of the circulartube 32 which comprises the central tube of the three concentric tubes32, 33 and 34. The drainage of the processing liquid from the surface 21of workpiece 22 is down the outside of tube 32 as shown at 35. Theannular space 36 between tubes 32 and 33 forms a reservoir for theprocessing liquid 13 and the annular space 37 between tubes 33 and 34forms a reservoir for voc material 27. While not shown, processingliquid is pumped into tube 32 and returned via reservoir 37 via a pumpand filter arrangement as in FIG. 1. The wetted zone 31 on the workpiecesurface 21 is circular due to the geometry of the tube 32. This wettedzone is confined by absorption into the processing liquid 13 of vocvapors 27 emanating from the annular space 37.

FIG. 3 shows an embodiment of the invention in plan view, similar tothat of FIG. 1, wherein the wetted zone 38 is a long, thin linear zonedefined by issuance of processing liquid from a slit 39. This liquidflows down the outside of the member or die 40 forming the slot 39.

FIG. 4 shows an interferometer consisting of a laser or collimated lightsource 47, an imaging system 48 consisting of mirrors, lenses, and otheroptics, capable of producing a fringe pattern 49 useful for imageprocessing.

FIG. 5 shows an apparatus consisting of a processing fluid applicator60; a fluid delivery system 61 consisting of a reservoir, pump andfilter; a voc source 62; a workpiece 63; a mounting structure for theworkpiece 64; a metrology device such as an interferometer 65; acomputer controller 66; and an XY translation stage 67 controlled bysaid controller.

Workpieces can include but are not limited to large silicon wafers, flatpanel display substrates, large glass sheets or optical substrates,crystal surfaces, metal sheets, etc. Uniform and constant irrigation bythe processing fluid in the immediate vicinity of the meniscus isessential for the successful operation of the Marangoni effect.

A volatile organic compound (voc) vapor source for establishment ofnecessary surface tension gradients can be a pool of liquid or asaturated porous material such as a sponge placed in the vicinity of themeniscus of processing fluid. It can also be entrained in a carrier gasflow delivered to the vicinity of the meniscus. The voc can be anycompound that has an appreciable vapor pressure at room temperature, iswater soluble, and has the effect that small amounts dissolved in watersignificantly reduce the water surface tension. Examples of suitablevoc's of relatively minor hazard include isopropanol and ethanol. Athermal means of establishment of necessary surface tension gradientscan be realized by heating the workpiece from the opposite side, or byintroducing a heat source such as an electrically heated wire or aheated gas flow surrounding the meniscus defining the perimeter of thewetted zone. To implement the motion of the wetted processing zone onthe workpiece surface, the workpiece can be moved relative to astationary processing fluid applicator, or the applicator can be movedrelative to the stationary workpiece, or both can be in relative motion.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention, whichis intended to be limited by the scope of the appended claims.

What is claimed is:
 1. An apparatus for etching or depositing a desiredprofile onto the surface of an object comprising: A. a means fordelivery of liquid etchant/deposition solution to an applicator, B. anapplicator configured for contacting a surface of an object with saidsolution, with a desired geometry, such that a free-surface flow ofliquid solution exists around a periphery of a wetted zone on theobject, C. means for inducing a surface tension gradient (Marangonieffect) in a meniscus region of a three-phase contact line where theliquid solution attaches to the object being processed, D. a linear orbi-directional motion stage for controlled motion of the wetted zonerelative to the surface of the object being processed, E. an encoder orinterferometer, imaging device and imageprocessing algorithm tocalculate topography of the object being processed in real time as theobject is in motion relative to said applicator, and F. acomputer-control system that uses the calculations obtained from thismetrology to control the motion of the stage.
 2. The apparatus of claim1, wherein said means for delivery of the liquid solution includes amember having an opening selected from the group consisting of anorifice and a slit.
 3. The apparatus of claim 1, wherein said applicatorincludes a plurality of concentric tubes.
 4. The apparatus of claim 1,wherein said applicator comprises a member having a central opening andtapered outer surfaces adjacent an end of said central opening.
 5. Theapparatus of claim 1, wherein said means for introducing a surfacetension gradient includes a reservoir containing a material capable ofproducing an organic vapor.
 6. The apparatus of claim 1, additionallyincluding means for recirculating said liquid solution.
 7. The apparatusof claim 1, wherein said applicator includes a housing having a centralopening an surrounded by a reservoir containing a material for producingan organic vapor.
 8. The apparatus of claim 7, wherein said housing andsaid reservoir comprise a plurality of concentric tubes, an intermost ofsaid concentric tubes defining said central opening.
 9. A flat glasscomprising the object and having a thickness of <1 mm processed by themethod of claim
 2. 10. The apparatus of claim 1, wherein the objectcomprises a flat glass sheet having a final thickness of <1 mm.
 11. Anapparatus for etching or depositing a desired profile onto a surface ofan associated object, comprising: a mechanism for delivering of liquidetchant/deposition solution to an applicator, said mechanism including amember having an opening selected from the group consisting of anorifice and a slit, said applicator being configured for contacting asurface of such an associated object with said solution, with a desiredgeometry, such that a free-surface flow of liquid solution exits arounda periphery of a wetted zone on such an associated object, means forinducing a surface tension gradient (Marangoni Effect) in a menicusregion of a three-phase contact line where the liquid solution attachesto such an associated object being processed, a motion stage forcontrolled motion of the wetted zone relative to the surface of such anassociated object being processed, an imaging device and imageprocessing algorithum to calculate topography of such an associatedobject being processed in real time as such an associated object is inmotion relative to said applicator, and a computer-control system thatuses the calculations from this metrology to control the motion of thestage.
 12. The apparatus of claim 11, wherein said motion stage isselected from the group consisting of linear and bi-directional motionstages.
 13. The apparatus of claim 11, wherein said imaging device isselected from the group consisting of an encoder in an aninterferometer.
 14. The apparatus of claim 11, wherein said applicatorincludes a plurality of concentric tubes.
 15. The apparatus of claim 11,wherein said applicator comprises a member having a central opening andtapered outer surfaces adjacent an end of said central opening.
 16. Theapparatus of claim 11, wherein said means for introducing a surfacetension gradient includes a reservoir containing a material capable ofproducing an organic vapor.
 17. The apparatus of claim 11, additionallyincluding means for recirculating said liquid solution.
 18. Theapparatus of claim 11, wherein said applicator includes a housing havinga central opening and surrounded by a reservoir containing a materialfor producing an organic vapor.
 19. The apparatus of claim 18, whereinsaid housing and said reservoir comprise a plurality of concentrictubes, an intermost of said concentric tubes defining said centralopening.
 20. The apparatus of claim 11, wherein the associated objectcomprises a flat glass sheet having a final thickness of <1 mm.